i 
1905. | On Vegetable Assimilation and Respiration. 459 
A table is constructed, showing the amount of energy unutilised at various 
temperatures by different leaves, with such noontide illumination in sun and 
in shade as actually prevailed on a definite day. 
Were the COz in the atmosphere augmented moderately, then this would 
cease to be the limiting factor in the shade generally, and also in feeble 
sunlight. Temperature, unless unnaturally high, would then limit assimila- 
tion in Nature, and still prevent bright insolation producing its full effect. 
[Notes added July 25, 1905. While this paper has been in the press, there has been 
published in this Journal an extensive paper on a cognate subject by Dr. Horace Brown 
and Mr. Escombe, entitled “The Physiological Processes of Green Leaves, with Special 
Reference to the Interchange of Energy between the Leaf and its surroundings” (‘ Roy. 
Soc. Proc.,’ vol. B 76, 1905, pp. 29 to 111). The following notes are now added to 
indicate as briefly as possible the points of contact between that paper and our own. 
As a point of general significance it must be noted that the expression “ amount of 
assimilation” has not the same meaning in the two papers. We use the unqualified words 
to mean the total photosynthetic work done in any time, part of the CO, for this being, 
of course, drawn from the external air and part from the concurrent respiratory activity 
of the leaf. Brown and Escombe intentionally leave respiration entirely out of account, 
and the unqualified word “assimilation” with them refers only to the intake of CO, from 
without. j 
A. (see p. 412). Brown and Escombe have published detailed balance sheets (pp. 100 to 
111) of the “Thermal Interchange” between leaves and their surroundings under various 
conditions of natural illumination. One interesting feature of these tables is the 
estimation of the internal temperature of the leaf. The degree of accuracy of this 
estimation is dependent upon the degree of accuracy of the six following determinations : 
(1) the coefficient of absorption of radiation by the leaf ; (2) the specific heat of the leaf ; 
(3) the energy being expended in transpiration ; (4) the energy being expended in 
photosynthesis ; (5) the thermal emissivity of the leaf; and (6) the effect upon this 
function of the particular velocity of movement of the surrounding air at the time. The 
temperatures arrived at by these truly admirable calculations are, however, in no case 
checked by actual determinations. The temperatures given in these tables for leaves in 
the sun in the open air are never more than 2° C. above the shade temperature of the air, 
while our few direct measurements with cherry-laurel leaves, brilliantly insolated, 
indicated 7° to 16° C. above the thermometer in the shade. Our method is not 
unexceptionable, but it does not seem probable that this divergence is to be wholly 
accounted for by the combined effects of faults of method, exceptional insolation, and 
thickness of leaf used. (Brown and Escombe employed only types of leaf which were 
thin.) 
B (see p. 485). Our @ priori criticism of the method of fractioniug light by the use of 
“rotating sectors” has not the cogency given to it in the text. Consideration of a 
number of experiments, in which this method was employed by Brown and Escombe, 
allows one to see that the leaf behaves in assimilation as the eye does in vision, and that 
halving the light im te¢me has, with all strengths of illumination, the same effect as halving 
it in intensity. The interest of the matter therefore shifts to the determination of the 
induction period and to finding at what slowness of rotation of the sectors the leaf begins 
to distinguish between the two methods of reduction of illumination. 
C (see p. 454). Brown and Escombe have made special inquiry into the trustworthiness 
of Sachs’ “increase of dry weight method” of estimating assimilation, and their verdict is 
very unfavourable. 
